fix output size adjustment for onnxifi_op
Summary: this breaks if we cut the net at certain int8 ops boundary.
Test Plan: with net_runner to lower a single Int8Quantize op. It used to break. Now it works.
Reviewed By: yinghai
Differential Revision: D22912178
fbshipit-source-id: ca306068c9768df84c1cfa8b34226a1330e19912
